Power converters, such as AC to DC converters or DC to AC inverters, may comprise networks of parallel and/or series connected power switching devices such as insulated gate bipolar transistors (IGBTs). Such converters may be for applications ranging from low voltage chips, to computers, locomotives and high voltage transmission lines. More specific example applications are switching in high voltage dc transmission lines of the type which may, for example, carry power from an offshore wind installation, and medium voltage (for example greater than 1 KV) switching for motors and the like, for example locomotive motors.
A converter may comprise one or more controller(s), e.g., intelligent devices which determine the required state of a collection of power switching devices. Furthermore, the converter may comprise switching units, such as intelligent “gate drives” which control the state of an individual power-switching device. Sensors such as temperature or current sensors, and/or actuators such as a cooling system pump may further be provided. Examples of the power switching devices include bipolar devices such as IGBTs, field effect transistors (FETs) such as MOSFETS (vertical or lateral) and JFETs, and potentially devices such as LILETs (lateral inversion layer emitter transistors), SCRs and the like. The techniques described herein are however not limited to any particular type of general converter architecture or any particular type of power switching device.
The use of such devices connected in parallel may be desirable to provide the required output power. Such an arrangement may allow a modular solution whereby power output can be scaled around a common platform. Additionally or alternatively, benefits may include enhanced performance through lowered parasitics in a given topology, lower cost, and/or very high power output particularly at high voltage.
Considering operation of such a parallel arrangement, however, variances between devices and drive parameters (e.g., IGBTs and corresponding gate drive parameters; the gate drive comprising circuitry to drive the IGBT gate terminal) may mean that it is not sufficient merely to switch each device in response to an incoming signal, e.g., PWM signal from a central controller. Such simple switching control may lead to poor current sharing/balance between the parallel devices, for example if the timing variance is greater than a few 10s of nanoseconds, and this may affect reliability. In this regard, it is noted that some timing uncertainty may be introduced by skew on conventional command interfaces to the gate drives, in particular where high voltage isolation is required between the central controller and the gate drives, and this can similarly degrade synchronism of device switching. Similarly, where inductances are not completely balanced between each of the parallel connected devices, the voltage may not change synchronously on each device. This is highly likely when the separation of devices such as IGBT modules is large (due to their physical size).
Where conventional, simple switching control is implemented, the prospect of current imbalance generally leads designers to de-rate the devices by, e.g., 10-20%; the derating margin may however be higher for faster switching devices. Consequently, IGBT module users may employ more modules than necessary for a given application.
Thus, the field of power switching device control continues to provide a need for methods for providing advantage(s) such as, inter alia, improving current sharing between parallel connected power switching devices, cost, size, bill of materials (e.g., number of power switching device modules), reliability, and/or power consumption for a given application, etc.
For use in understanding the present invention, the following disclosures are referred to:
Decentralized Active Gate Control for Current Balancing of Parallel Connected IGBT Modules—Paper; Y. Lobsiger, et al—2011; available at http://www.pes.ee.ethz.ch/uploads/tx_ethpublications/15_Decentralized_Active_Gate_EPE2011.pdf; and
Active Gate Control for Current Balancing of Parallel-Connected IGBT Modules in Solid-State Modulators—Paper; Johann W. Kolar, et al—2008; http://www.pes.ee.ethz.ch/uploads/tx_ethpublications/bortis_IEEETrans_ActiveGate.pdf.